164 publications from this institution
Abstract Prokaryotic CRISPR-Cas immune systems detect and cleave foreign nucleic acids. In type III CRISPR-Cas systems, the Cas10 subunit of the activated recognition complex synthesizes cyclic oligoadenylates (cOAs), second messengers that activate downstream ancillary effector proteins. Once the viral attack has been weathered, elimination of extant cOA is essential to limit the antiviral response and to allow cellular recovery. Various families of ring nucleases have been identified, specializing in the degradation of cOAs either as standalone enzymes or as domains of effector proteins. Here we describe the ring nuclease activity inherent in the SAVED domain of the cA4-activated CRISPR Lon protease CalpL. We characterize the kinetics of cA4 cleavage and identify key catalytic residues. We demonstrate that cA4-induced oligomerization of CalpL is essential not only for activation of the protease, but is also required for nuclease activity. Further, the nuclease activity of CalpL poses a limitation to the protease reaction, indicating a mechanism for regulation of the CalpL/T/S signaling cascade. This work is the first demonstration of a catalytic SAVED domain and gives new insights into the dynamics of transcriptional adaption in CRISPR defense systems.
Electron ionisation mass spectra of N ‐methylimidazolidin‐4‐one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five‐membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Abstract PN bond activation of 2 H ‐azaphosphirene complexes 1 and 2 by using triflic acid led to ring expansion in the presence of nitriles. In the absence of nitriles, the reaction surprisingly afforded two haptomeric N‐protonated 1‐aza‐3‐phospha‐butadiene complexes in the case of complex 1 , whereas the N‐protonated 2 H ‐azaphosphirene complex [H‐ 2 ] + was characterized by NMR spectroscopy. magnified image Protonation of 2 H ‐azaphosphirene complexes 1 and 2 using trifluoromethanesulfonic acid in the presence and in the absence of nitriles was studied. Reactions with nitrile derivatives of different nucleophilicity and steric demand ( 3 a – f ) led to highly bond‐ and regioselective ring‐expansion reactions of complexes 1 and 2 and yielded 2 H ‐1,4,2‐diazaphosphole complexes 4 a – f and 5 a after deprotonation with NEt 3 . Upon protonation of complex 1 in the absence of nitriles a mixture of two haptomeric N‐protonated 1‐aza‐3‐phosphabuta‐1,3‐diene complexes 13 and 14 was obtained, which upon addition of Me 2 NCN ( 3 a ) and NEt 3 afforded 2 H ‐1,4,2‐diazaphosphole complex 16 a bearing a P‐substituent with only one SiMe 3 group. Protonation of complex 2 led to the 2 H ‐azaphosphirenium complex [H‐ 2 ] + , unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR, MS, and single‐crystal X‐ray crystallography in the cases of complexes 4 a , 4 e , 6 a , and 16 a . Furthermore, ESI‐MS/MS studies on primarily formed reactive intermediates and DFT studies on the reaction mechanism are presented.
Abstract A detailed mass‐spectrometric study provides insight into the gas‐phase fragmentation pathways of a cyclic helicate selectively built from four iron(II) centers and six [2.2]cyclophane‐based ligands through the subcomponent self‐assembly approach. The charge state of the precursor ion, i. e., the number of triflate anions accompanying the metallo‐supramolecular core, has a strong influence on the observed fragmentations. The triply charged ion shows loss of a neutral ligand whereas ions of higher charge fragment by up to three different charge‐separating pathways to minimize the charge density of the ions. Additional subsequent fragmentations of highly charged fragment ions include redox processes as well as splitting of the unusual paracyclophane backbone.
